A Passive Optical Network, PON is a point-to-multipoint network architecture employing fibre cables from a Central Office, CO, to premises. It employs unpowered optical splitters to enable a single optical fibre to serve multiple premises. A PON comprises an Optical Line Terminal, OLT, at the CO of the service provider. It comprises a number of Optical Network Terminals, ONTs, near end users. A PON configuration reduces the amount of fiber and CO equipment required compared with point-to-point architectures. A PON is a form of fiber-optic access network.
In order to supervise and monitor the performance of a PON, Optical Time-Domain Reflectometry, OTDR, may be used. The supervision and monitoring of PONS using OTDR is further described in the recommendations L-25, L-40, L-42 and L-53 from the Telecommunication Standardization Sector of International Telecommunication Union (ITU-T). Briefly described, an OTDR device injects a series of optical pulses into the fiber. The series of optical pulses, also called OTDR signal(s) travel down the network towards the ONTs. Parts of the OTDR signals are reflected back towards the OTDR device. The back reflected, or backscattered, OTDR signal may be used for estimating the fiber's length and overall attenuation, including losses such as splitter losses. The backscattered OTDR signal may also be used to locate faults, such as breaks, and to measure optical return loss.
However, the amount of light scattered back to the OTDR for measurement is quite small, about one-millionth of the optical power in the test pulse. Since so little of the light comes back to the OTDR for analysis, the OTDR receiver circuit must be very sensitive. That means that big reflections, which may be one percent of the outgoing signal, will saturate the receiver, or overload it.
Another difficulty with OTDR measurements is that backscattered light from the fibers between a splitter and the ONTs will be summarized passing back through the splitter and again attenuated, making it very difficult to obtain the magnitude of a possible fault in one or more fibres between the splitter and the ONTs.
In order to customize and adapt the layout of a PON to the end-users' needs, more advanced architectures are now emerging. For example, tree architectures, may be achieved by adding sub-splitters to the drop links. Normally, the sub-splitter divides a drop link into a tree comprising two or more of drop links branches, i.e. drop links form the sub-splitter to the ONT. Thereby, more drop links may be added. However, adding a sub-splitter may make it even more difficult to obtain the location and magnitude of a fault in the PON.
As a consequence of the difficulties described above, misinterpretations may arise when reading or analyzing the backscattered OTDR signal, also referred to as the trace, after the splitter or the sub-splitters. Even if a fault is detected after the splitter, i.e. between the splitter and the ONTs, it is impossible to define which drop link or sub-splitter, i.e. which fiber between the splitter and the sub-splitter/ONTs, is affected as the received backscattered signal is a superposition of power coming back to the splitter from all of the drop links.